This invention relates to electro-optic devices and, more particularly, to proximity coupled light valves for electro-optic line printers and the like.
It has been shown that an electro-optic element having a plurality of individually addressable electrodes can be used as a multi-gate light valve for line printing. As is known, almost any optically transparent electro-optical material can be used as the electro-optic element of such a light valve. The most promising materials now appear to be LiNbO.sub.3 and LiTaO.sub.3, but there are other materials which qualify for consideration, including BSN, KDP, KD.sup.x P, Ba.sub.2 NaNb.sub.5 O.sub.15 and PLZT. In any event, the electrodes of such a light valve are intimately coupled to the electro-optic element and are distributed in non-overlapping relationship widthwise of the electro-optic element (i.e., orthogonally relative to its optical axis), typically on equidistantly separated centers so that there is a generally uniform interelectrode gap spacing. The electrodes of an electro-optic device, such as a multi-gate light valve, may be fabricated on a suitable substrate and pressed against or held very close to the electro-optic element to provide what is referred to as "proximity coupling".
To perform line printing with a multi-gate light valve of the foregoing type, a photosensitive recording medium, such as a xerographic photoreceptor, is exposed in an image configuration as it advances in a cross line direction (i.e., a line pitch direction) relative to the light valve. More particularly, to carry out the exposure process, a sheetlike collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection. Furthermore, successive sets of digital bits (hereinafter collectively referred to as "data samples"), which represent respective collections of picture elements or pixels for successive lines of the image, are sequentially applied to the electrodes. As a result, localized electric bulk or fringe fields are created within the electro-optic element in the immediate vicinity of any electrodes to which non-reference level data samples are applied. These fields, which are coupled into the electro-optic element through an air gap in proximity coupled devices, in turn, cause localized variations in the refractive index of the electro-optic element within an interaction region (i.e., a light beam illuminated region of the electro-optic element which is subject to being penetrated by the electric fields). Thus, the phase front or polarization of the light beam is modulated (hereinafter generically referred to as "p-modulation" of the light beam) in accordance with the data samples applied to the electrodes as the light beam passes through the interaction region. Schilieren readout optics may be used to convert a phase front modulated light beam into a light beam having a correspondingly modulated intensity profile. For example, the phase front modulated light beam may be imaged onto the recording medium by central dark field or central bright field imaging optics. Alternatively, if the input light beam is polarized, a polarization modulation to intensity modulation conversion process may be performed by passing the polarization modulated output beam through a polarization analyzer. In more generic terms, the p-modulation of the light beam is converted into a correspondingly modulated intensity profile by using "p-sensitive readout optics" to image or project (hereinafter collectively referred to as imaging) the light beam onto the recording medium.
In order to use only one electrode per picture element, a differential encoding scheme may be used. With such a scheme, the intensity pattern to be recorded across a scan line is differentially encoded so that any picture element to be turned on is represented as a voltage difference between two neighboring electrodes independent of sign, and any picture element to be turned off is represented as a common voltage between two electrodes independent of magnitude.